With the evolution of the third generation (3G) network and the increase of the number of wireless transmit/receive units (WTRUs) being used, operators are pursuing solutions for new services with higher data rates at reduced costs. An introduction of home Node B (HNB) is considered as a viable candidate solution and is currently being studied as part of third generation partnership project (3GPP) Release 8. The HNB offers services over relatively small service areas, such as home or office. It has a similar role to a wireless local area network (WLAN) access point (AP). The service may be provided where cellular coverage is poor or non-existent. A subscriber typically will own the HNB and have control over its final deployment location. The backhaul link from the HNB to the network may be achieved through the conventional access technologies, (e.g., digital subscriber line (DSL), cable modem, wireless, etc.). A set of HNBs are connected to a HNB gateway (GW) that provides an interface to the core network.
A subscriber is able to configure the HNB as its “desired” or “preferred” HNB by using the concept of a closed subscriber group (CSG). A CSG identifies WTRUs that are permitted to access the HNB. An HNB cell may be configured to only allow access from a set of WTRUs that are part of the CSG. This has implications for initial access registration during WTRU power up, or roaming while in an idle mode. Although legacy (pre-Release 8) mobility management signaling can be used to address the access control problem, it is not efficient. Once a WTRU camps on a cell or reselects a cell, the WTRU sends a location area (LA) update message to a network mobility management entity every time the WTRU changes an LA. The LA is broadcast in the non-access stratum (NAS) specific information within the system information block 1 (SIB1) message. If the WTRU is not part of the CSG for the HNB cell, the network will respond with a location area reject message. The WTRU may then add this LA to its “forbidden LA” list.
An idle mode WTRU may make multiple registration attempts (in vain) as the WTRU roams through an area with a high concentration of HNBs that do not have the WTRU in their CSG. This results in an increased signaling load and should be avoided.
With respect to cell reselection, a WTRU in the vicinity of its “preferred” HNB should reselect the preferred HNB cell. This implies that some mechanism must be in place to encourage this cell reselection. For 3GPP Release 7 and earlier, cell reselection measurements on neighbour cells are only triggered when the quality level of the current cell (S) is not met for Nserv DRX cycles, or is less than Sintrasearch, Sintersearch, or SsearchRATm. These parameters are broadcast as part of the system information. This poses a problem if the WTRU should always select its preferred HNB cell if it is in its vicinity. In particular, additional mechanisms are required to trigger the measurements and once triggered, the HNB cells should be favoured. In addition, as the penetration of HNBs could be quite high, a method is needed to restrict the amount of measurements that WTRUs make in cells that are inaccessible.
All WTRUs may need to be allowed to camp on a CSG cell in order to make an emergency call. With respect to localization, an operator should be able to know the location of the HNBs in order to provide emergency services.
Another issue with respect to the HNBs is that a WTRU in URA_PCH state will send UTRAN routing area (URA) update messages when the WTRU crosses a URA boundary. URAs typically encompass a group of adjacent cells, and are defined to reduce the amount of reselection signaling as WTRUs move across cells. The network knows the location of the WTRUs at the URA level and pages a WTRU across all cells belonging to the URA. In addition, each cell may belong to more than one URA. The list of URAs is broadcast from the cell as part of the system information. For HNB deployments, the HNBs are not tied to a physical location and the user or owner of the HNB has some flexibility in terms of placement of the HNBs. Therefore, the scheme of a cell belonging to a static list of URAs may not apply in the HNB scenario. In addition, the flexible HNB deployment may also imply that idle mode paging procedures can be improved.